A seat lifter apparatus is usually provided to a seat for a vehicle in order to adjust a height of the seat according to a physical attribute or preference of a seat occupant. A known seat lifter apparatus disclosed in JP2009-154638A includes a four-bar linkage which is constituted by a base, a front lift link, a lower arm and a rear lift link attached to the lower arm by means of a pin. The rear lift link is formed with a sector gear for engaging with a pinion that is driven and rotated by a motor. The four-bar linkage is driven and operated by the pinion, thereby moving the lower arm upward and downward. The sector gear includes a no-teeth portion, where no teeth are formed, at each circumferential end of the sector gear. The pinion stops rotating when the pinion mounts on the no-teeth portion, which serves as a stopper.
A lifter mechanism disclosed in JP2006-282019A also employs a for-bar linkage. According to the lifter mechanism, a pinion gear and a sector gear both positioned at a front portion drive a rear link via a link rod. Upward and downward movements are stopped by means of a contact between the pinion gear and a no-teeth potion, where no teeth are formed, formed at the sector gear, and by means of a contact between a long hole formed on a seat side frame and a connecting pin fixedly provided at the sector gear.
According to the seat lifter apparatus disclosed in JP2009-154638A, however, when the pinion stops the rotation, a contact portion between each of the no-teeth portions of the sector gear and a tooth of the pinion comes to be under a large load. In this large load, a component force acting in a radial direction of the sector gear (that is, in a direction in which the pinion and the pin are separated from each other) is larger than a component force acting in a circumferential direction of the sector gear. The teeth of the pinion or a bearing portion of a rotation shaft of the pinion may be worn out under a repetitive application of the large load, possibly causing a tooth clatter. In particular, in case that the seat lifter apparatus is operated downward when an occupant is seated on the seat, a weight of the occupant also acts downward in addition to a driving torque of the pinion driven by the motor, which may cause even larger load.
As with the seat lifter apparatus disclosed in JP2009-154638A, according to the lifter mechanism disclosed in JP2006-282019A, a force is applied to a contact portion between the pinion gear and the sector gear in a direction in which an axis of the pinion gear and an axis of the sector gear are separated from each other. In addition, a contact portion between the long hole and the connecting pin comes to be under a reaction force against the contact therebetween. However, it is often difficult to distribute the above-mentioned two forces to above-mentioned two contact portions evenly at the same time, and either one of the two contact portions may receive an excessive force. In case that the excessive force acts to the pinion gear and to the sector gear, a similar phenomenon may occur to that in JP2009-154638A. In case that the excessive force acts to the long hole and to the connecting pin, the long hole or the connecting pin may be deformed.
A need thus exists for a seat adjusting apparatus for a vehicle, which is not susceptible to the drawback mentioned above.